Field of the Invention
This invention relates to motor control circuits, which could also be considered as motor energization circuits. More particularly, the invention is directed to, and has for an object provision of, a circuit effective to insure that the controlled motor shall draw essentially the same magnitude motor-energizing current when it starts up as it draws while in steady-state running condition, and no greater than the running current.
The invention has application in a motor/blower system using series-field-type (or simply "series") universal motors. Such a motor characteristically draws a very high starting current (in a conventional energization circuit), which may be up to ten times its running current.
In a working embodiment of the invention an alternating current universal series motor was employed. However, the invention could be utilized with direct current series field type motors, and in applications other than motor/blower systems, i.e. in other applications where limitation of starting current is desirable.
In arriving at the present invention, several approaches were considered, but had to be rejected for reasons discussed below. Solid state silicon-controlled-rectifier control or triac control had to be ruled out because of excessive electromagnetic interference generated by switching transients.
Another approach was to place a negative temperature thermistor in series with one of the motor leads. The thermistor chosen was capable of handling the currents (5 amp per motor) and limiting the starting current to the maximum running current. However, the time constant of the thermistor did not sufficiently match the startup time (or "starting time constant") of the motor and its load, here the blower. The lack of success in this approach led to the successful approach of the invention.